The present invention relates to an optical element. More particularly, the invention relates to an optical element in which many structures formed of projections or depressions are arranged on the surface thereof at a fine pitch equal to or smaller than a wavelength of visible light.
Conventionally, there have been known optical elements including light-transmissive substrates composed of glass, plastic, or the like, which are subjected to surface treatment in order to suppress surface reflection of light. As such surface treatment, a method is known in which fine and dense projections and depressions (moth eyes) are formed on the surface of an optical element (for example, refer to “OPTICAL AND ELECTRO-OPTICAL ENGINEERING CONTACT”, Vol. 43, No. 11 (2005), 630-637).
In general, in the case where a periodic projection-depression shape is provided on a surface of an optical element, when light passes through the surface, diffraction occurs, and thereby the amount of the light component that goes straight, of transmitted light, is significantly reduced. However, in the case where the pitch of the projection-depression shape is shorter than the wavelength of light transmitted, diffraction does not occur, and it is possible to obtain an effective anti-reflection effect for single-wavelength light corresponding to the pitch, depth, or the like of the projection-depression shape.
As the moth-eye structure fabricated using electron beam exposure, a moth-eye structure in the shape of fine tents (pitch: about 300 nm, depth: about 400 nm) is disclosed (for example, refer to NTT Advanced Technology Corporation, “Master mold for forming anti-reflective (moth-eye) structure having no wavelength dependency”, [online], [searched on Feb. 27, 2008], Internet <http://keytech.ntt-at.co.jp/nano/prd_0033.html>). The moth-eye structure is, for example, believed to be fabricated as follows.
First, a projection/depression pattern is formed by electron beam recording on a photoresist on a Si substrate, and the Si substrate is etched using the projection/depression photoresist pattern as a mask. Thereby, tent-shaped, fine sub-wavelength structures (pitch: about 300 nm, depth: about 400 nm) are formed on the surface of the substrate. A Si master mold is thus fabricated (refer to FIG. 1A). The fine structures are arranged in a tetragonal lattice pattern or in a hexagonal lattice pattern.
The Si master mold thus fabricated can have an anti-reflection effect for light having a wide wavelength range. In particular, as shown in FIG. 1B, when tent-shaped, fine sub-wavelength structures are provided in a hexagonal lattice pattern, a high anti-reflection effect (reflectivity: 1% or less) can be obtained in the visible region (refer to FIG. 2). In FIG. 2, symbols l1 and l2 respectively indicate the reflectivity of the flat portion and the reflectivity of the patterned portion of the Si master mold.
Next, a Ni-plated stamper of the resulting Si master mold R1 is produced (refer to FIG. 3). As shown in FIG. 4, a projection/depression pattern reversed from the projection/depression pattern of the Si master mold is formed on the surface of the stamper. Next, using the stamper, the projection/depression pattern is transferred to a transparent polycarbonate resin. Thereby, an intended optical element (replica substrate) is obtained. The optical element can also have a high anti-reflection effect (reflectivity: 1% or less) (refer to FIG. 5). In FIG. 5, symbols l3 and l4 respectively indicate the reflectivity in the absence of the pattern and the reflectivity in the presence of the pattern.
However, electron beam exposure is disadvantageous in that it requires a long operation time, and is unsuitable for industrial production. Formation of the projection/depression pattern by the electron beam recording and the area that can be exposed depend on the amount of current of electron beam and the dose amount necessary for the resist. For example, in the case where drawing is performed, using a beam of 100 pA, which is used in drawing the finest pattern, on a resist requiring a dose amount of several tens of microcoulombs per square centimeter, such as a calixarene, even if exposure is performed for 24 hours, a square with a side of 200 μm cannot be filled. Furthermore, it takes 25 days or more to expose a square of 1 mm×1 mm, and exposure is believed to be limited to a microdevice with a size of several hundred micrometers or less.
Meanwhile, in the case where drawing is performed, using a beam of 2 nA, which is not substantially increased, on a chemically-amplified resist that can be exposed at about 100 μC/cm2 or less, such as SAL601 or NEB-22, a square of 2 mm×2 mm can be drawn in one hour or less. Note that the required dose amount varies depending on the substrate/development conditions, etc. In general, a high dose amount is suitable for high resolution.
However, even in this production method, it requires a considerably large number of days to expose a small display size, thus being inefficient, which is disadvantageous. For example, it takes 50.8×38.1/(2×2)=483.9 hours (about 20 days) to expose an area of a mobile phone with a small display (2.5 inch; 50.8 mm×38.1 mm), which is currently commonly used.
The Super-RENS Technology Team, the Center for Applied Near-Field Optics Research of the National Institute of Advanced Industrial Science and Technology (hereinafter referred to as “AIST”), has succeeded in the development of a nano-fabrication apparatus on the basis of a thermal lithography technique in which a visible-light laser lithography method using a semiconductor laser (wavelength: 406 nm) and a thermally nonlinear material are combined (for example, refer to the National Institute of Advanced Industrial Science and Technology, “Development of a Desktop Apparatus Enabling Nanometer-scale Microfabrication”, [online], [searched on Feb. 27, 2008], Internet <http://aist.go.jp/aist_i/press_release/pr2006/pr20060306/pr20060306.html>).
A technique of high-speed recording on a disc substrate with a diameter of 12 cm has been being developed. Utilizing characteristics of the high-speed/low-cost/large-area fabrication technique of optical discs, AIST and Pulstec Industrial Co., Ltd. have been jointly working on development of optical elements having a nanometer-scale fine structure (moth-eye low-reflection structure) which can be fabricated at high speed with a large area and in which the cost can be reduced, and development of apparatuses.
The thermal lithography technique in which a visible-light laser lithography method and a thermally nonlinear material are combined is a method which utilizes a temperature distribution occurring in a light spot. When a substance is irradiated with light, if the substance has a light-absorbing property, light energy is converted to heat. Light focused by a lens on a substrate has a Gaussian light intensity distribution, and distribution of heat generated owing to absorption of light by the substance has a similar temperature distribution profile.
Consequently, by using, as a light-absorbing material, a material that rapidly changes owing to heat generated by absorption of light, it is possible to realize fine lithography in a size equal to or smaller than the diameter of the light spot. In this method, when a change in volume of a substance is caused in a minute region of a photoresist by thermochemical reaction or thermal diffusion of the substance to perform lithography, it is difficult to fabricate structures with a resolution of 100 nm or less and a high aspect ratio, and reproducibility is also difficult to achieve. Under these circumstances, a new material and process technology have been reexamined, and a thermal lithography technique which can reliably reproduce structures of 100 nm or less with high aspect ratios has been developed. Thereby, a desktop nanometer-scale microfabrication apparatus has been completed.
The nanometer-scale microfabrication apparatus includes a rotational stage, a uniaxial stage, and an auto-focus unit, which enables nanometer-scale, high-speed lithography. Furthermore, a semiconductor laser with a wavelength of 405 nm is used for laser beams for drawing, and an objective lens with a numerical aperture (NA) of 0.85 is used in an optical system for focusing light, thereby realizing a very compact apparatus.
FIG. 6 shows a nano-dot pattern formed by the apparatus having the configuration described above. The result shown in FIG. 6 is obtained by irradiation of blue pulsed laser light while rotating at a rate of 6 m/s (2,600 to 3,600 rpm) to perform drawing. By driving a laser beam at a pulse frequency of 60 MHz, the apparatus can form a dot pattern of 50 nm, which is equal to or less than one-sixth of the light beam spot size, at a rate of 6 million dots/s. The drawing rate of the ordinary electron beam lithography apparatus or the like is about 0.2 m/s, and therefore, the apparatus described above can form nanometer-scale fine structures at a speed 30 times higher than the ordinary apparatus. Furthermore, by combining this technique with a dry etching method used in the semiconductor process, it is possible to form a nano-hole structure with a diameter of 100 nm and a depth of 500 nm or more over the entire surface of a substrate with an optical disc size (diameter: 12 cm). In such a manner, using the apparatus described above, it is possible to fabricate a mold for nano-imprinting having a nanometer-scale fine pattern over a large area at high speed and low cost.
Furthermore, FIG. 7 shows an example in which a fine structure including a SiO2 disc substrate with a diameter of 12 cm having an anti-reflection function is fabricated to reduce light reflectivity. Although it is possible to fabricate the anti-reflective nano-structure at high speed/large area/low cost, the reflectivity is close to 2%, and thus, this structure is not a non-reflection structure, but is a low-reflection structure.
The reason for the fact that a low-reflection structure is produced is believed to be that the density (aperture ratio) the nano-holes is low (50% or less) and the Fresnel reflection at the plane other than the nano-holes is high. In contrast, as shown in FIGS. 1A and 1B, when tent-shaped nano-structures are formed in a closest packing, hexagonal lattice pattern, a non-reflective effect can be realized.